1. Field of the Invention
The invention relates to an improved design for a heater blanket for use in in-situ decontamination of soil.
2. Description of the Prior Art
Assignee's U.S. Pat. No. 4,984,594 to Vinegar et al discloses an in-situ apparatus and method for remediation and decontamination of surface and near-surface soils by evacuating the soil under a flexible sheet, which is impermeable to gases, and heating the soil surface with a relatively flat electric surface heater, which is permeable to gases. This patent discloses that the surface heater may be a mesh constructed from a conducting metal, such as stainless steel or resistance wire sold under the trademark NICHROME, which can tolerate temperatures as high as 1000.degree. C. or more. The mesh has a total effective resistance of several ohms and good mechanical strength for durability when dragged over rough terrain. A 325 mesh of 304 stainless steel with wire 0.0014-inch in diameter is preferred. This patent discloses that the surface heater may also be constructed of metal wires which loop back and forth but do not form a mesh. For example, the metal wires could be stitched into a ceramic fiber cloth such as wire-reinforced NEXTEL 440 a trademark of Minnesota Mining & Manufacturing Co. of St. Paul, Minn. The surface heater could also consist of metal strips or ribbons or a metallic coating in a defined pattern. Still another possibility is to construct the surface heater from a thin metal foil or sheet with perforations to allow gas flow through the foil or sheet.
A preferred configuration for a permeable mat with good thermal insulation propeties, according to the patent, is a layer of NEXTEL ceramic fiber combined with a layer of corrugated 304 stainless steel knitted wire mesh with about 1/4-inch crimp. The knitted wire mesh has extremely high gas permeability and can easily support a force of 15 pounds per square inch. Moreover, the thermal conductivity of such a knitted wire mesh is low. The permeable mat may be composed of multiple layers, including insulating layers, to reduce convection and radiation. Thus, the permeable mat may comprise a a layer of NEXTEL ceramic fiber, then a layer of corrugated knitted wire mesh, then a layer of perforated aluminum foil for a radiation reflecting shield, then another layer of NEXTEL, another layer of knitted wire mesh, another aluminum foil shield, etc.
In Assignee's co-pending U.S. patent application Ser. No. 800,192 filed Nov. 27, 1991, an improved heater blanket for use in soil remediation utilizes a rigid construction which isolates the heater elements from air, water, and contaminant vapors which could act to oxidize, embrittle or otherwise degrade the integrity of the electrical heaters. A rigid structural frame comprises a support frame of parallel members which are rigidly connected to a series of support tubes perpendicular thereto. The support tubes enclose and protect the heater elements. One or more heater modules thus constructed may be easily transported to a selected site. A heater element, preferably stranded wire, is placed inside an electrically insulating sleeve, preferably NEXTEL. The sleeve and the wire are placed inside a support tube, preferably made of stainless steel. An array of tubes is rigidly connected to rigid frame members for ease of assembly/transportation. The spacing between the tubes may be 1-6 inches with bus bars connecting the wires from each individual tube to a source of electrical power.